1. Field of the Invention
The present method and apparatus relates generally to positioning systems for wireless user equipment, and more specifically to a wireless system that examines user equipment cellular identification and cellular identification age.
2. Relevant Background
Accurate position information of user equipment (UE) such as cellular telephones, personal communication system (PCS) devices, and other mobile stations (MSs) is becoming prevalent in the communications industry. Global Positioning Systems (GPS) offer an approach to providing wireless UE position determination. GPS employs satellite vehicles (SVs) in orbit around the earth that transmit positioning signals. A UE equipped with a GPS receiver can derive precise navigation information including three-dimensional position, velocity and time of day through information gained from the SV transmissions.
One disadvantage of the GPS system for location determination is the relatively long time needed to perform signal acquisition under certain conditions. SV signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and observed Doppler frequency shift. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case after a receiver “cold start”, a large number of code delays and frequencies must be searched for each SV signal that is to be acquired and tracked. These locations are examined sequentially, a process that can take several minutes in a conventional GPS receiver.
In order to reduce this delay, information may be provided to aid a GPS receiver in acquiring a particular signal. Such assistance information permits a receiver to narrow the search space that must be searched in order to locate a signal, by providing bounds on the code and frequency dimensions. The predicted code window provides a reduced range within which the “code phase” (effectively, the signal time of arrival, or “pseudorange”) should be found, or a predicted range of observed Doppler shift associated with the signal. Narrower code and frequency windows reduce the overall search space resulting in a reduction in the time in which the receiver takes to acquire the signal. A system that employs a GPS receiver augmented with externally sourced GPS assistance data is commonly referred to as an “assisted global positioning system” (AGPS).
The goal of GPS position assistance information is to permit the UE to predict the time of arrival, or code phase, of a particular SV signal, and the Doppler shift of the SV signal. If the UE is provided with an initial reference position that is within an area of predefined size, such as a particular cellular coverage, then the total search space can be reduced to that consistent with the predefined size.
One example of an AGPS system is wireless UE with GPS capabilities in communication with one or more base stations (BSs), also referred to as base transmitting stations (BTSs), or node Bs, which in turn communicate with one or more servers, also called Position Determination Entities (PDEs) or Serving Mobile Location Centers (SMLCs) depending upon the communication air interface protocol. The PDE derives GPS assistance information from one or more GPS reference receivers.
The PDE has access to a means of determining the approximate UE position. This might consist of a “base station almanac” (BSA) that provides BTS/node B location based upon a serving cell identification (ID) reported by the UE. The BSA provides the approximate location of the UE by providing the geographical coordinates for a reference position based upon the unique serving cell ID, while the PDE computes the signal acquisition assistance information customized for the approximate UE position. Alternatively, this information may be derived via network procedures and Mobile Application Part (MAP) standards, such as via an Any Time Interrogation (ATI) request by the network to the “home location registry” (HLR) associated with the UE. The HLR is a database that stores user subscription and identity information. In that instance, the network obtains the cell ID through network procedures such as submission of an ATI request that may contain the international mobile equipment identity (IMSI) or mobile station integrated services digital network (MSISDN) identification for the UE.
The cell ID is extracted by the UE from periodically broadcast system information messages from the BTS. However, during lengthy communications when changing cells or base stations the UE does not necessarily retrieve the cell ID of subsequent BTSs, but instead retains the cell ID of the BTS to which the UE connected during power-up. Thus the cell ID “ages” during communication. For more rapid identification purposes, when a UE is handed off to a subsequent BTS, the smaller, locally-unambiguous BSIC identifier—in GSM protocol, or PSC identifier—in UMTS protocol, along with the carrier frequency is accessed by the UE. When the cell ID known to the UE has aged, UE position uncertainty increases. If the age of the cell ID known to the UE is relatively young, then the cell ID is likely accurate and position is more certain.
Another limitation of obtaining a cell ID for the purpose of providing position assistance information to the UE occurs when network-based procedures are used to obtain cell ID, such as when the network initiates an ATI request to the HLR. This process requires valuable network resources to obtain the cell ID when instead the UE may be capable of providing current cell information, particularly if the cell ID age is young.
A need exists for a method and apparatus that optimizes network resources by reducing dependence upon frequent network-initiated ATI procedures to obtain a UE cell ID when a current cell ID may be available from the UE itself. The current cell ID can then be used in determining UE position information.